If you want to stay longer than 90 days within a 180 days period, you need a national long-term (type D) visa. Even if your 90/180-visa (type C?) is still valid, it should at least in theory be possible to "upgrade" it to a type D visa. If you are going to Italy to work, you will also need a work permit. I am note sure if you can apply for that together with the visa, or if there are separate application processes.
A national type D visa gives you the right to a long term stay in the issuing country and in practice the same rights as a type C visa in the other Schengen countries. You are allowed to transit and visit other Schengen countries within the same limits as the citizens of the issuing country. This may be a problem if you need to transit other Schengen countries to get to Italy. Even if you get a new type D visa from Italy, I would assume that your 90 days are considered to be used and that the new visa in this case won't give you the right to enter or transit other Schengen countries.
You should however discuss this with the Italian consulate and be able to get any necessary details on what you are entitled to with a new visa, if they should issue one to you.